Method and apparatus for treating biologically contaminated air

ABSTRACT

A method for reducing biological contamination of indoor air, which method includes providing a concentrated salt solution, which is either (i) a brine capable of responding to aeration thereof by a rapid increase of its Redox potential, wherein the rate of said increase is greater than the rate of increase observed for a 45% (w/w) calcium chloride solution subjected to identical aeration conditions; or (ii) a brine which is passed through an electrolytic cell in order to raise its Redox potential; circulating said concentrated salt solution through a treatment zone; causing a stream of biologically contaminated air to flow through said treatment zone, such that said contaminated air is contacted with said salt solution in said treatment zone; and withdrawing purified air from said treatment zone.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of International ApplicationNo. PCT/IL2006/001010, filed 31 Aug. 2006, which designated the U.S.,and claims priority to Israel Application No. 170605, filed 1 Sep. 2005,the entire contents of each of which are hereby incorporated byreference.

Commonly used methods for treating contaminated air include varioustypes of filtration, air ionization and sterilization of air utilizingozone or ultraviolet light as disinfectants.

Air filters, such as activated carbon filters and High EfficiencyParticulate Air (HEPA) filters, are used to remove various chemicals,odors and dust from a stream of air passing therethrough.

Air ionization involves the generation of electrically charged ions inthe air. The negatively charged ions become attached to floatingparticles, which eventually fall out of the air.

Ozone-based purifiers involve the formation of ozone molecules bygenerating a high-voltage electrical discharge through air or oxygen, orby using some types of ultraviolet lamps. Ozone is known as an excellentoxidizing agent and bactericide.

Ultraviolet (UV) air purifiers are based on the disinfecting propertiesof UV light. These purifiers contain lamps which generate UV lightcapable of destroying germs, viruses and bacteria.

Some of the purification systems used today combine several of the aboveindicated methods to effectively remove particulates and destroymicrobial contaminants. For example, in a commercially availableapparatus “XJ-3000 C” of Heaven Fresh Canada Inc(http://www.heavenfresh.ca/catalog/), the purification of air isaccomplished utilizing: i) air ionization; ii) HEPA air filtration; iii)germicidal UV lamp; iv) an activated carbon filter; v) ozonepurification (optional); and vi) anti-bacterial pre-filter for removinglarge particles (for prolonging the efficiency of the carbon and HEPAfilters).

It may be appreciated that among the various types of contaminated airthat needs to be treated and purified, indoor air in hospitals, requiresa special attention. Common measures currently used for preventing, orat least reducing, the spread of infections in health care facilitiesinclude Heating, Ventilating and Air Conditioning (HVAC) maintenance andcleaning, increased ventilation, Ultraviolet Germicidal Irradiation(UVGI), and air filtration. Ideally, a successful treatment of indoorair in hospitals should bring a significant decrease in the level ofmicrobiological load in large volumes of air using inexpensive means.

EP 0230875 describes a process for treating a contaminated air using afilter provided with layers that contain alkaline and acidic agents.

WO 01/78868 describes a process for purifying air by passing a stream ofair over rough surfaces covered with salt micro-crystallizedsedimentation.

L. Øvreås et al. [“Characterization of microbial diversity inhypersaline environments by melting profiles and reassociation kineticsin combination with terminal restriction fragment lengthpolymorphism(T-RFLP)”, Microb Ecol. 2003 October; 46 (3):291-301. Epub2003 Aug. 14] report that the total genetic diversity of prokaryoticcommunities at different salinities (22, 32, and 37%) were increasedfrom 22% to 32% salinity and were reduced at 37% salinity to nearly halfthat at 22% salinity.

US 2004/0231512 describes a method and a device for conditioning air andpurifying the same by contacting the air with a liquid desiccant (andspecifically, a solution of lithium chloride, lithium bromide or calciumchloride), which are commonly used in air conditioning systems asdehumidifiers. The liquid desiccant is subsequently regenerated byheating the same.

It has now been found that certain brines, which are capable ofresponding to a passage of air therethrough by a rapid and significantincrease of their Redox (RedOx—Reduction Oxidation) potential, may beeffectively used for reducing considerably the level of biologicalcontaminates present in an indoor air, upon being contacted with thecontaminated air. More specifically, it has been observed that the Redoxpotential of certain brines is rapidly and significantly raised afterthe brine has been adequately contacted and mixed with the stream of airto be purified, following which the resulting brine becomes a powerfuldisinfectant.

It has now also been found that oxidant brines, which in the context ofthe present invention are brines that are periodically passed through anelectrolytic cell in order to increase their Redox potential, may alsobe effectively used in the purification of biologically contaminatedair.

Accordingly, the purity and quality of indoor air that it is suspectedof being biologically contaminated (e.g., interior hospital air), may besignificantly improved following treatment with certain concentratedbrines or with oxidant brines.

In a first aspect, the present invention provides a method for reducingbiological contamination of indoor air, which method comprises:

providing a concentrated salt solution, which is either (i) a brinecapable of responding to aeration thereof by a rapid increase of itsRedox potential, wherein the rate of said increase is greater than therate of increase observed for a 45% (w/w) calcium chloride solutionsubjected to identical aeration conditions; or (ii) a brine which ispassed through an electrolytic cell in order to raise its Redoxpotential; Circulating said concentrated salt solution through atreatment zone; causing a stream of biologically contaminated air toflow through said treatment zone, such that said contaminated air iscontacted with said salt solution in said treatment zone; andWithdrawing purified air from said treatment zone.

The term “brine” is used herein to indicate concentrated, nearlysaturated or saturated salt solutions, namely, solutions wherein theconcentration of the salt dissolved therein is preferably between 20%,and more preferably between 30% (w/w) and up to saturation at therelevant temperature.

The term “biologically contaminated air” is used herein to indicate thatthe air referred to contains clinically (or otherwise) undesired levelsof bacteria, mycoplasma, protozoa, viruses (specifically Polio and Adenoviruses) and/or other types of microorganism. The term “purified air” isused herein to indicate that the level of microorganism in the airrecovered by the method of the present invention has been reduced in ameasurable quantity in comparison to the pre-treated air, preferably byat least 5%, and more preferably by at least 25% and most preferably byat least 50%.

The Redox potential of the concentrated salt solution is preferably notless than 200 mV, and more preferably not less than 300 mV, and mostpreferably not less than 400 mV during at least a portion of the time itis contacted with the biologically contaminated air. The Redoxpotentials reported herein are measured using Pt/Ag/AgCl arrangement,thus indicating the electrochemical potential which is developed betweenPt electrode exposed to the brine and a standard silver-silver chlorideelectrode.

Two classes of concentrated salt solutions are suitable for use in thepurification of the biologically contaminated air according to thepresent invention.

The first class of concentrated salt solutions includes brines which arecapable of spontaneously developing high (e.g., above 300 mV) oxidationcapacity in-situ, by simply contacting a stream of air to be purifiedwith said brine, following which the brine is transformed into apowerful disinfectant, capable of considerably reducing the level ofmicroorganisms in said stream of air. It should be noted that typicalliquid desiccants, namely, a commercially available alkaline solution oflithium chloride (or lithium bromide), or a solution of calciumchloride, do not develop a sufficiently high Redox potential in responseto a passage of air therethrough.

The second class of concentrated salt solutions to be used according tothe present invention encompasses brines whose Redox potential is raisedby passing the brine through an electrolytic cell.

More specifically, the first class comprises concentrated salt solutionswhich satisfy the following property: when the solution is subjected toaeration and the variation of its Redox potential is measured againsttime during said aeration period, then the rate of increase of saidRedox potential is greater (preferably by more than 20%) than thecorresponding rate measured, under identical aeration conditions, for a45% (w/w) calcium chloride solution. The rate of variation of a Redoxpotential with time may be readily determined by plotting the same as afunction of time, obtaining the derivative of said function andcalculating its value for a certain point at time. However, forpractical purposes, the variation of the Redox potential with time maybe approximated by a suitable function, whose derivative represents therate of variation of the Redox potential with time. Thus, most simply,the rate variation of the Redox potential of a solution is referred toherein as the difference between the Redox potential measured at twodifferent points at time, which points are designated t₁ and t₂, dividedby the difference in time (t₂−t₁). Most conveniently, t₁ is taken as thestarting point, before the solution is subjected to aeration (t=0), andt₂ may be chosen to be 60 minutes, 120 minutes, 180 minutes, 240 minutesor 300 minutes.

A simple set-up for determining the behavior of the Redox potential of agiven salt solution in response to aeration thereof, and hence, itssuitability for use in accordance with the present invention, isillustrated in FIG. 1. It should be noted that this set-up is providedas an example, and alternative arrangements may be used in order tocollect the data required for plotting the Redox potential of givenbrine against time.

The set-up comprises an open vessel 100 having a volume of 12 liters, aspray head 101 (for example, in the form of a common shower head)positioned above said open vessel and connected thereto by means of apipe 102. A pump 103 is used to circulate the solution in the device.

The solution is pumped from vessel 100 and returned thereto throughspray head 101. The distance between the spray head 101 and the vesselis 20 cm and the number of openings in the spray head is 30 per 100 cm².The downwardly directed streams generated by spray head 101 are ofcourse exposed to air, and hence the solution is aerated. The devicefurther comprises a pair of electrodes 104 (namely, a working electrodemade of platinum and a reference electrode (Ag/AgCl)) suitable formeasuring a Redox potential of a solution contacted therewith, whichelectrodes are conveniently located upstream. Typically, the volume ofthe solution is in the range of 10 to 12 liters and it is allowed tocirculate within the set-up described above at a rate of about 1 to 1.5liter/min.

The initial Redox potential of a concentrated salt solution is typicallyin the range between 20 and 120 mV. The Redox potential of a saltsolution circulated in the device varies (increases) in time, since thesolution is contacted with air and is mixed therewith. During thisaeration period, the Redox potential of the solution is measured at timeintervals of about 15 to 30 minutes, and the results are plotted againsttime.

The set-up described in FIG. 1 was used to determine the variation withtime of the Redox potential of two typical liquid desiccants, and theresults are shown by means of bar diagrams in FIGS. 2 a and 2 b, for a45% (w/w) calcium chloride solution and an alkaline 45% (w/w) lithiumchloride solution, respectively. It may be seen that after an aerationperiod of about 120 minutes, the Redox potential of the calcium chloridesolution reaches a value of about 180 mV. The average rate of increaseof the Redox potential of the calcium chloride solution during thisperiod of aeration is calculated as described above [(180 mV−50 mV)/120min], and is approximately 1 mV/min. The average rate of increase of theRedox potential of the alkaline solution of the lithium chloridedesiccant is calculated similarly (120 mV−20 mV)/120 min, and is about0.8 mV/min for the first two hours of aeration of the liquid desiccant.

In contrast with the desiccants referred to above, the preferredconcentrated salt solutions to be used according to the presentinvention are capable of developing a Redox potential of not less than300 mV after having been circulated in the set-up described in FIG. 1for 120 minutes. The average rate of increase of the Redox potential ofthe preferred solutions to be used according to the invention istherefore not less than 1.5 mV/min, and more preferably not less than2.0 mV/min for the first two hours of aeration of the salt solutionunder the conditions of the set-up described in FIG. 1. Furthermore,under said conditions, the Redox potential of a salt solution suitablefor use according to the invention exceeds 400 mV following an aerationperiod of 180 min.

Compositionally, the concentrated salt solution belonging to the firstclass identified above, which may be used according to the presentinvention for disinfecting the biologically contaminated air, is anaqueous solution containing one or more water soluble salts representedby the formulas MX, M₂X and MX₂, wherein X is selected from the groupconsisting of chloride, bromide, iodide, sulfate and nitrate anions, andM indicates a metal cation, which is most preferably selected from thegroup consisting of sodium, potassium, calcium, magnesium and zinc, withthe proviso than when X is chloride and M is other than zinc, then thesolution comprises at least two water soluble salts. The presence ofbromide and/or iodide anions in the concentrated salt solution isespecially preferred.

A preferred concentrated salt solution, which may be used fordisinfecting the biologically contaminated air, comprises zinc bromideor zinc chloride, or a mixture thereof, at a weight concentration of 40%to 55%. FIG. 3 is a bar diagram illustrating the variation with time ofthe Redox potential of 55% (w/w) zinc bromide and 55% (w/w) zincchloride solutions. The data was collected using the set-up described inFIG. 1. It is apparent that the Redox potential of both zinc halidesolutions increases rapidly with time. For Example, the average rate ofincrease of the Redox potential of the zinc bromide solution followingtwo hours of aeration is greater than 3.0 mV/min.

Another preferred concentrated salt solution to be used according to theinvention comprises a mixture of at least one bromide and/or iodidesalt, and at least one chloride salt of one or more of the followingmetals: Na⁺, K⁺, Mg²⁺ and Ca²⁺. An especially preferred solutioncontains a mixture of bromide and chloride salts dissolved therein in atotal concentration of 30 to 40% by weight, with the cationic speciesbeing Mg²⁺, Ca²⁺, Na⁺ and K⁺. More specifically, the concentrations ofthe aforementioned ions are as follows: Mg²⁺: 30-50 g/liter; Ca²⁺: 10-20g/liter; Na⁺: 30-50 g/liter; K⁺: 5-10 g/liter; Cl⁻: 150-240 g/liter;Br⁻: 3-10 g/liter. An example of such a solution is provided by the DeadSea brine, which has the following typical (average) mineralcomposition: Mg²⁺: about 40.6 g/liter; Ca²⁺: about 16.8 g/liter; Na⁺:about 39.1 g/liter; K⁺: about 7.26 g/liter; Cl⁻: about 212.4 g/liter;Br⁻: about 5.12 g/liter, with the total concentration of salts dissolvedtherein being 33% by weight.

A particularly preferred concentrated salt solution comprises a mixtureof bromide and chloride salts dissolved therein in a total concentrationof 30 to 40% by weight, with the cationic species being Mg²⁺, Ca²⁺, Na⁺and K⁺, wherein the concentration of calcium chloride in said solutionis effective in reducing the rate of evaporation of water therefrom, andis preferably in the range between 100 and 200 g/liter. When used in thepurification process of the present invention, this especially preferredsolution was found to develop a Redox potential of above 450 mV,combined with a very slow rate of evaporation of water therefrom.

FIG. 4 is a bar diagram which shows the variation with time of the Redoxpotential of a Dead Sea brine, having the composition describedhereinabove, to which calcium chloride was added (in an amount of 150g/liter). It may be seen that the solution comprising the Dead Sea brineand an additional amount of calcium chloride develops a Redox potentialgreater than 300 mV following two hours of aeration, and a Redoxpotential greater than 450 mV following four hours of aeration.

It has been observed that the Redox potential of the specific brinesidentified above, namely, the zinc halide brines and brines containing amixture of at least one bromide salt and one chloride salt, such as theDead Sea brine, is raised to above 300 mV after the brine has beencirculated in the treatment zone in accordance with the presentinvention for a relatively short period of time (less than two hours),during which period it has been contacted and aerated with a stream ofthe biologically contaminated air. Thus, rather paradoxically, themedium to be purified according to the present invention, namely, astream of biologically contaminated air, raises the Redox potential ofthe brine belonging to the first class (as defined above) which is mixedtherewith, to such an extent that an in-situ aerated brine is generated,having strong disinfecting ability, such that said in-situ formed brinemay be circulated within the treatment zone, continuously purifying thecontaminated air flowing therethrough.

The second class of concentrated salt solutions to be used according tothe present invention encompasses brines whose Redox potential isperiodically raised by passing the brine through an electrolytic cellsuch that oxidants are generated in the brine, for example, chlorine,hypochlorite and hypochlorous acid, thus affording an oxidant brine,which is subsequently contacted with the air to be treated. Oxidantbrines produced by means of electrolysis which may be used according tothe present invention have a Redox potential of about 200-370 mV, andpreferably 250-370 mV. The oxidant brine may be generated by passing aconcentrated solution of one or more halide salts through anelectrolytic cell. The term “halide salts”, as used herein, specificallyincludes salts which contain a cation selected from the following group:Li⁺, Na⁺, K⁺, Mg²⁺ and Ca²⁺. It has been surprisingly found that anoxidant brine of sodium chloride, having a concentration of 30 to about40% by weight, functions as a powerful disinfectant under the conditionsof the present invention.

It should be noted that the concentrated salt solution belonging to thefirst class identified above, namely, those solutions whichspontaneously develop a high Redox potential (above 300 mV or even above400 mV) in response to aeration thereof, may also be passed throughelectrolytic cell, if desired, in order to further intensify theiroxidation capacity.

Accordingly, in a preferred embodiment, the present invention provides amethod for treating a biologically contaminated air, which comprisescausing a stream of biologically contaminated air to flow through atreatment zone, contacting said contaminated air with a concentratedsalt solution circulating through said treatment zone and recovering apurified air therefrom, wherein the circulation of said solutioncomprises introducing the solution into the treatment zone, collectingthe solution in a suitable vessel after it has been in contact with thecontaminated air, driving the solution from said vessel and raising theRedox potential of the solution by passing said solution through anelectrolytic cell, and reintroducing the solution into the treatmentzone, in order to purify biologically contaminated air flowingtherethrough.

The circulation of the concentrated salt solution, as describe above,may be conveniently carried out at ambient temperature; there is nonecessity to heat the circulated salt solution.

The Redox potential of the brine is a readily measurable property, whichmay serve for monitoring the operation of the method provided by theinvention. The measurement of the Redox potential is convenientlyachieved by contacting the circulating brine, or a sample of the brine,with a suitable set-up, which set-up typically comprises a measuringelectrode made of an inert metal or alloy (a platinum electrode) and areference electrode (such as Ag/AgCl or calomel) connected to avoltmeter.

Depending on the results of the Redox potential measured by the set-upset forth above, or an alternative set-up, the Redox potential may beadjusted, e.g., by electrolyzing the brine, or enhancing the rate of airflow through the treatment zone, whereby the Redox potential isincreased. Thus, the method provided by the present invention mayfurther comprise periodically or continuously measuring the Redoxpotential of the brine solution, and adjusting the Redox potential ofthe brine based on the measured value of the Redox potential. Underroutine mode of operation, the brine solution circulating within thetreatment zone preferably has Redox potential in the range between 200and 450 mV. Thus, if the measurement of the Redox potential indicatesthat the Redox potential is lower than 150 mV, then the electrolyticcell may be activated, in order to pass electrical current through thebrine solution, whereby oxidant species are formed therein. As indicatedabove, certain brine solutions are capable of developing high Redoxpotential in response to enhanced aeration conditions. Accordingly, forthese preferred brines as set forth above, the adjustment of the Redoxpotential may be accomplished by appropriately controlling the flow ofair through the treatment zone.

The set-up which serves for measuring the Redox potential of the brineis positioned at any appropriate location in the pathway of thecirculating brine, for example, inside the reservoir holding the brine,or in a conduit used to deliver the brine into the treatment zone. Themeasured Redox potential may then be used to provide one or moreautomatic feedback signals to the electrolytic cell or to the meansserving for feeding the air into the treated zone, in order to adjusttheir operation. Alternatively or in addition, the electrolysis of thebrine can be adjusted by a human operator based on the observed Redoxpotential. To this end, the measurement of the Redox potential may beused to generate an alarm signal, to trigger the interference of thehuman operator once the measurement of the Redox potential indicates avalue outside a specified working range. Alternatively or in addition,an alarm signal may be generated once the measurement of the Redoxpotential indicates unacceptably low value (e.g., below 100 mV).

The term “electrolytic cell”, as used herein, refers to a set-upcomprising electrodes which are electrically connected to the oppositepoles of a direct electrical current (DC) power supply. The electrolyticcell is placed in any suitable location in the pathway of thecirculating brine. For example, two suitable electrodes are affixedwithin the reservoir used for storing the brine solution, oralternatively, within a conduit used to transfer the brine to thetreatment zone. The electrodes are preferably placed in parallel to eachother, separated by a gap of 0.3 to 2.0 cm, and more preferably of 0.5to 1.0 cm. The electrodes are preferably in the form of plates or mesheshaving a length and a width of about 2 and 5 cm, respectively. Theelectrodes are generally composed of a material selected from the groupconsisting of titanium (coated with ruthenium oxide), platinum or analloy of platinum and iridium. The cell typically operates at a currentdensity of 10³-10⁵ Ampere per square meter of anode, applying a voltagein the range between 2 and 10 V, and preferably about 3-5 V. Within thebroad range of conditions described above, the electrolysis of a brinesolution having a volume of about 10-100 liter may be carried out forabout 5-10 minutes, with a current of few Amperes, following which thebrine solution attains an operative Redox potential. It may beappreciated that the duration of the electrolysis may depend on the typeand concentration of the brine used and the aeration conditions.

In the event that there is a need to decrease the Redox potential of thebrine solution (for example, should the measured value exceeds 450 mV),then the brine is chemically treated with an effective amount of one ormore oxidizer-scavenging compounds. The term ‘oxidizer-scavengingcompounds’ is used herein to indicate organic and inorganic compoundswhich are useful in removing oxidizers (e.g., oxygen, halogens,oxyhalogens) from an aqueous solution. Oxidizer-scavenging compoundswhich act as reducing agents, and specifically, sulfur-based reducingagents, such as water soluble salts of sulfite, bisulfite, thiosulfate,metabisulfite, hydrosulfite or mixtures thereof, as well as otherreducing agents such as ascorbic acid, are all within the scope of thepresent invention. The reducing agent may be fed into the oxidant brinein a solid or in a liquid form (e.g., as an aqueous solution). Themeasured Redox potential may then be used to provide one or moreautomatic feedback signals to a container which holds the solution ofthe reducing agent, in order to deliver suitable quantities of saidreducing agent into the brine. For example, when the volume of the brinesolution employed in the method of the present invention is between 10and 100 liters, then about few milliliters of a solution of sodiumbisulfite, or sodium thiosulfate, having a concentration of about 5%(w/v) may be used in order to decrease the Redox potential of the brine.

According to a particularly preferred embodiment, the method of thepresent invention comprises passing the stream of contaminated airthrough or onto one or more surfaces provided within the treatment zone,wherein said one or more surfaces are wetted by a concentrated saltsolution circulating through said treatment zone, thereby increasing theliquid surface area in contact with the air, and recovering a purifiedair from said treatment zone. Most suitably, the treatment zonecomprises one or more layers of absorbent material, small glass balls,ceramic rings or PVDF (poly vinilyden difluoride) rings, which arecapable of providing high active surface area. According to oneembodiment, the surfaces serving as the contact area for the air and thebrine are in the form of layers made of absorbent fibrous material,which layers are wetted by one or more streams of a concentrated saltsolution, wherein said streams flow in the treatment zone in an oppositedirection to the flow of the contaminated air. The aforementionedarrangement of porous surfaces made of absorbent layers (e.g., cellulosefibers) that are mounted within the treatment zone, in combination withcountercurrents of air and brine flowing therein, generates an increasedliquid surface area in direct contact with the air to be treated,allowing the brine to carry out its disinfection action effectively.

In another aspect, the present invention provides an apparatus suitablefor reducing the biological contamination of indoor air, whichcomprises:

-   -   a chamber having at least first and second openings for        receiving a stream of biologically contaminated air and for        withdrawing purified air therefrom;    -   means for causing a stream of air to flow into said chamber and        outwardly therefrom through said first and second openings,        respectively;    -   a basin located beneath said first opening;    -   a pump and one or more conduits connecting said basin to the        interior of said chamber, wherein said pump and conduit(s) are        suitable for withdrawing a brine from said basin and feeding the        same into the interior of said chamber;    -   a first pair of electrodes positioned in said one or more        conduit(s), wherein said electrodes are electrically connected        to the opposite poles of a direct electrical current (DC) power        supply; and    -   a second pair of electrodes positioned in said one or more        conduit(s), capable of measuring the Redox potential of a brine        passing therethrough.

Means for causing a stream of air to flow into the chamber and outwardlytherefrom may be a fun, a suction pump or a centrifuge compressor airsupply. Most preferably, one or more surfaces capable of increasing thecontact area between air and liquid are mounted within the chamber. Forexample, the chamber is preferably provided with a fill having highactive surface area, typical to fibrous materials or small glass ballsor ceramic rings, or PVDF (poly vinilyden difluoride) rings. Accordingto a specific embodiment, the chamber has a fill assembly positionedtherein, which assembly is in the form of a three-dimensional matrixcomposed of a plurality of grids spaced from one another and disposed inperpendicular planes to form essentially cubic spaces therebetween, intowhich spaces absorbent fibrous material may be introduced. The grids maybe made of polypropylene and have a thickness of about 0.5 mm to 1 mm.The side of the cubic space that is filled with the fibrous material, asmentioned above, may vary in the range of 3 to 5 cm. In operation, theair to be purified and the brine are brought into intimate contactwithin the surfaces made by said fibrous absorbent material, whichsurfaces are being supported by means of said fill assembly within thechamber.

FIG. 5 schematically illustrates an apparatus 10 for effectivelyreducing microbiological load in air according to a preferred embodimentof the invention. Apparatus 10 comprises an elongated, verticallypositioned treatment zone 17 (for example, an open counterflow tower)having a first opening 32 and a second opening 33 in the lowermost anduppermost sections thereof, for receiving an upward stream ofbiologically contaminated air 15, and for withdrawing a purified air 14from said tower, respectively. A fan 11 (or a suction pump) fitted atthe upper opening 33 is adapted to generate a negative pressure insidethe tower 17, thus producing air suction, causing a stream ofbiologically contaminated air (designated by arrows 15) to flow upwardlyin tower 17 and outwardly therefrom. The fan 11 generally provides airsuction rates of about 2000 cubic meters/hr.

The tower 17 further comprises a plurality of sprinkles or spray heads23 mounted in an upper section thereof. The brine is drawn from a basin22 positioned below opening 32 by means of a fluid pump 21, and isdelivered into the interior of tower 17 by means of a suitable ductwork28 and the aforementioned sprinkles or spray heads 23 connected thereto.Streams of brine 24, generated by said sprinklers, flow downwardlyinside tower 17 and are collected in basin 22, thus allowing thecirculation of the brine.

The dimensions and shape of tower 17 may be readily selected by theskilled artisan. In general, the cross section area of tower 17 is inthe shape of a square or a circle. The walls of tower 17 may be made ofvarious materials such as plastic, polyethylene, polypropylene,polyester, fiberglass or suitable stainless steel (AISI 304, 316, 410,420 and 430). In general, it has been found that a brine solutioncirculating at a rate of 3 to 15 m³/hr is capable of effectivelypurifying a stream of air drawn by the fun at a rate of 400 to 2000m³/hr.

The interior of tower 17 serves to contact the brine 24 sprinkled at theupper section of said tower with the stream of the biologicallycontaminated 15 driven upwardly in the tower and outwardly therefrom, byfan 11. In this way, the microorganisms contaminating the air areexposed to the saline and undergo proteins and DNA denaturation whichdisables their ability to reproduce and thus shorten their life span. Aswill be discussed hereinbelow, this operation when continuously carriedout in closed spaces (e.g., buildings), substantially reduces the amountof indoor airborne microbiological contamination therein.

Tower 17 preferably comprises one or more absorbent layers (designated18 a, 18 b, . . . , 18 n) disposed therein, such that the upwardlydirected air stream 15 is forced to pass through said layers. Absorbentlayers 18 may be positioned essentially perpendicularly to thelongitudinal axis of tower 17, as illustrated in the figure. Theselayers may be affixed to the internal walls of the tower by conventionalmeans (not shown). Alternatively, a fill assembly that is conventionallyused in cooling towers may be placed in tower 17, and the spaces of saidfill assembly may be loaded with the absorbent material. Absorbentlayers 18 are wetted by the downwardly flowing streams 24 of the brineand become impregnated therewith, thus allowing improved contact betweenthe contaminated upwardly directed air stream 15 passing therethroughand the brine 24. Absorbent layers 18 are generally made of cellulosefibers and have a thickness in the range of 3 to 7 cm, preferably about5 cm. It has been found that the preferred number of absorbent layersprovided within tower 17 is 3 to 5. It should be noted that in order toincrease the contact surface area between the liquid and the air to betreated, tower 17 may be provided with means conventionally employed forsuch purposes, for example, Rasching rings, made of ceramic material orplastic material such as PVDF (poly vinylidene difluoride) or PP(polypropylene) or PE (polyethylene) that are commonly used indistillation columns.

Apparatus 10 further includes an electrolytic cell (EC), 30 forgenerating an oxidant brine having a desired Redox potential. In itsmost simple configuration, an electrolytic cell suitable for useaccording to the present invention comprises two electrodes that areaffixed within conduit 28 and placed in parallel to each other,separated by a distance of 0.3 to 2.0 cm, and more preferably of 0.5 to1.0 cm. The electrodes are preferably in the form of plates or mesheshaving a length and a width of about 2 and 5 cm, respectively. Theelectrodes are generally composed of a material selected from the groupconsisting of titanium (possibly coated with ruthenium oxide), platinumor an alloy of platinum and iridium. The electrodes are electricallyconnected to the opposite poles of a direct electrical current (DC)power supply.

In operation, the brine pumped from basin 22 passes in the electrolyticcell 30, which operates under 2 to 3.5 V, at a rate of about 3 to 15m³/hour, whereby the brine is electrolyzed. Upstream there is provided aRedox potential measurement device (RDX) 31, for determining the Redoxpotential of the oxidant brine. Such a device includes a workingelectrode made of platinum, exposed to the stream of the optionallyelectrolyzed brine, and a conventional reference electrode (Ag/AgCl).The brine having a Redox potential in the range of 200 to 500 mV, andmore preferably 300 to 450 mV, flows into tower 17 and is contactedtherein with an upwardly directed stream of biologically contaminatedair 15. The brine then exits tower 17 by force of gravity throughopening 32 and is being collected in basin 22, from which it is recycledin the apparatus.

In one specific preferred embodiment of the invention apparatus 10further comprises a control unit 35 electrically connected (designatedby dotted arrowed lines) to fan 11, pump 21, electrolytic cell 30, andRedOx potential measurement device 31. Control unit 35 is adapted formonitoring and managing the operation of apparatus 10 responsive tosignals received from RedOx potential measurement device 31, inparticular, the operation of electrolytic cell 30 employed forelectrolyzing the brine solution passing in ductwork 28 duringoperation, and the operation of fan or suction pump 11 for adjusting therate of contaminated air 15 flowing into apparatus 10.

Optionally, apparatus 10 further comprises key pad 35 k and display unit35 d (e.g., dot matrix or LCD) electrically connected to control unit35, for receiving inputs from an operator, and for providing theoperator output indications regarding the operation of apparatus 10. Ofcourse, apparatus 10 may comprise additional means connected to controlunit 35 for generating output indications (e.g., leds, speakers).

Control unit 35 may be implemented by a specially designed control logiccircuitry, preferably by a programmable microcontroller. At least oneanalog to digital converter may be needed by control unit 35 forconverting the signals received from RedOx measurement device 31.

Apparatus 10 preferably further comprises a container 36, for holding asolution of oxidizer-scavenging compounds 36 c, wherein said containercommunicates with basin 22 through pipe 36 p. The passage through pipe36 p may be controlled by means of valve 36 v, electrically connected tocontrol unit 35, for controlling the feeding of the solution of theoxidizer-scavenging compounds into the brine 2 in order to reduce itsRedox potential.

Control unit 35 is preferably adapted for activating apparatus 10responsive to inputs received from key pad 35 k (or an on/off switch—notshown), by issuing respective control signals for activating pump 21 forpumping brine 2, and fan 11 for introducing contaminated air 15 streamin rates suitable for normal operation of apparatus 10 e.g., 1000 to1500 m³/hr. Control unit 35 is preferably adapted to adjust theperformance of apparatus 10 responsive to readings received from RedOxmeasurement device 31. In particular, the RedOx potential of brine 2 maybe adjusted by varying the rate of flow of contaminated air 15introduced into apparatus 10, such that control unit 35 may be adaptedto increase the rotation speed of fan 11, for increasing the rate ofcontaminated air stream e.g., 1500 to 2000 m³/hr introduced intoapparatus 10, responsive to low RedOx potential readings e.g., less than150 millivolts.

Additionally or alternatively, control unit 35 may be further adapted toactivate the electrolytic cell 30 responsive to low RedOx readings e.g.,less than 150 millivolts. On the other hand, if the measurement of thepotential indicates unacceptably high RedOx potential in brine 2, thencontrol unit may issue control signals for opening valve 36 v forintroducing a small quantity (e.g., drops) of the oxidizer-scavengingcompounds 36 c contained in container 36, whereby the brine solutionattains a normal Redox potential e.g., 200-400 millivolts.

It should be noted that the method and apparatus provided by the presentinvention may be effectively used for purifying air in many differentkinds of large closed spaces. In addition to hospitals, which werealready indicated hereinabove, poultry farms, greenhouses, tunnels,train stations and spaces exposed to heavy smoking may also bespecifically mentioned. The method and apparatus of the invention areuseful as a prophylactic measure for preventing avian flu, sickbuildings syndrome (SBS) and other viral diseases. Specifically, it hasbeen found that Dead Sea brine having the composition described above isuseful in significantly reducing the level of the followingmicroorganisms in the treated air:

Bacteria—E. coli, K. pneumonia, S. aureus, P. aeruginosa and Bacillusspecies.

Fungi—Aspergillus species.

Viruses—Polio Virus and Adeno Virus.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates an arrangement suitable for measuringthe Redox potential of a brine subjected to aeration.

FIGS. 2 a and 2 b are bar diagrams showing the Redox potential ofcalcium chloride and alkaline lithium chloride solutions as a functionof time.

FIG. 3 is a bar diagram showing the Redox potential of zinc halidesolutions as a function of time.

FIG. 4 is a bar diagram showing the Redox potential of a Dead Sea brineas a function of time.

FIG. 5 schematically illustrates an apparatus for effectively reducingthe amount of microbiological organisms residing in air;

FIG. 6 is bar diagram showing the reduction of microbial load in airfollowing treatment with a non-electrolyzed brine;

FIG. 7 is a bar graph showing the reduction of microbial load uponoperating the apparatus of the invention with a single absorbent layer;

FIG. 8 is a bar diagram showing the reduction of microbial load uponoperating the apparatus of the invention with two absorbent layers;

FIG. 9 is a bar diagram showing the reduction of microbial load uponoperating the apparatus of the invention with three absorbent layers;

FIG. 10 is a bar diagram showing the results obtained upon running theapparatus continuously.

FIGS. 11A to 11C are photos showing microbiological growth in TSA+5% SBsettle plates of samples taken from the incoming and outgoing airflowsof the apparatus of the invention operating with a single absorbentlayer;

FIGS. 12A and 12B are photos showing the development of yeast in SDAsettle plates containing samples taken from the incoming and outgoingairflows of the apparatus of the invention operating with a singleabsorbent layer; and

FIGS. 13A to 13E are photos showing microbiological growth in TSA+5% SBsettle plates of samples taken from the incoming and outgoing airflowsof the apparatus of the invention operating with two absorbent layers.

FIG. 14 is a bar diagram showing the results obtained upon running theapparatus of the invention using a Dead Sea brine.

FIG. 15 is a bar diagram which illustrates the air quality in a hospitaldepartment treated according to the invention, and a non-treateddepartment.

EXAMPLES

Examples 1 to 5 describe the results of tests which were carried outwith the apparatus illustrated in FIG. 5 in the 5^(th) floor of thebone-marrow department of the Chaim Sheba Medical Center inTel-Hashomer, Israel. The apparatus was placed in a corridor (equippedwith no air-conditioning means), and was operated over a month undervarious conditions, as discussed below. During the tests the corridorwas is air connection with the stairway and elevator system of themedical center. The volume of the corridor space was approximately 700m³.

The height of the apparatus was 200 cm, and the dimensions of itscross-section area were 50 cm×50 cm. The apparatus operated under thefollowing conditions:

The rate of air suction during the tests was 400 m³/H.

The brine used was an aqueous solution of sodium chloride having 35%salinity or a Dead Sea Brine. The brine was pumped at a rate of 10m³/hour.

The electrolytic cell included two titanium electrodes coated with analloy of ruthenium oxide, commercially available from Denura LTD, Italy.These electrodes were placed in parallel within the conduit connectingthe basin and the top of the tower. The gap between the electrodes wasof 1 cm.

The electrodes used to measure the Redox potential of the brine were aplatinum electrode and silver/silver chloride electrode, commerciallyavailable from Trytel IL.

The absorbent layers used are made of natural cellulose fibers. Eachlayer consisted of 4 cm thick fibrous material, placed in the lowersection of the tower, in the vicinity of the first (lowermost) openingthereof.

Air samples of the air entering (hereinafter incoming airflow) andleaving (hereinafter outgoing airflow) the apparatus were collected byan air sampler onto general microbial colonies TSA+5% SB (Tryptone SoyaAgar+5% Sheep Blood) settle plates and onto yeast and fungal coloniesSDA (Sabouraud Dextrose Agar) settle plates. The TSA+5% SB plates wereincubated at 37° C. for 24 to 48 hours, the SDA plates were incubated at37° C. for 24 to 48 hours, and the results are reported in ColonyForming Units (CFUs) per cubic meter.

Example 1 Comparative The Efficacy of a Non-Electrolyzed Brine as aDisinfectant for Biologically Contaminated Air

The apparatus illustrated in FIG. 5, containing two absorbent layersmounted therein, was operated without activating the electrolytic cell,as follows:

NaCl brine is pumped from the basin and is circulated through theapparatus. After 0.5 hour, the Redox potential of the circulating brineis periodically measured at intervals of 20 minutes and air samples (ofthe incoming contaminated airflow and the outgoing purified airflow) areconcurrently collected onto a TSA+5% SB settle plates. The bar diagramshown in FIG. 6 and the table below summarize the results obtained. Inthe accompanying bar diagrams, a pair of adjacent columns represents thelevel of microbial contamination (CFU/m³ units) measured in theincoming, contaminated airflow (right column) and the outgoing, purifiedairflow (left column). A pair of adjacent columns is designated by acapital letter or by the time at which the measurement was taken.

Microbial Microbial Redox contamination contamination potential incomingoutgoing Measurement (mV) air flow (CFU) air flow (CFU/m³) 1 140 340 1102 180 400 120 3 185 620 240

Example 2 The Effect of the Number of Absorbent Layers Provided withinthe Tower on the Efficacy of the Air Purification

The apparatus illustrated in FIG. 5, containing either one, two or threelayers of absorbent material horizontally deposited therein, is operatedas follows:

NaCl brine is pumped from the basin and passed through the electrolyticcell (3 V) to produce an oxidant brine having a Redox potential of about200 to 250 mV. The oxidant brine is circulated through the apparatus forabout 1 to 2 hours, during which period air samples (of the incomingcontaminated airflow and the outgoing purified airflow) are periodicallycollected onto a TSA+5% SB settle plates.

The bar diagrams shown in FIGS. 7, 8 and 9 summarize the resultsobtained for the use of one, two and three absorbent layers in thetower, respectively. These results are also indicated in the tablebelow:

TABLE 2 Microbial Microbial Number of contamination contaminationabsorbent incoming outgoing layers air flow (CFU/m³) air flow (CFU/m³) 1910 480 1 900 610 1 810 580 2 650 250 2 650 250 2 350 150 3 1220 310 31200 240 3 1350 380 3 620 140 3 430 175

Example 3

The apparatus was allowed to operate continuously, with 3 layers ofcellulose fibers provided therein. The Redox potential of thecirculating brine was about 230 mV. Air samples were collectedperiodically from both the incoming airflow and the outgoing airflow.FIG. 10 is a bar diagram showing the Colony Forming Units (CFUs) percubic meter determined for a sample taken from the incoming air and theoutgoing air (right and left columns, respectively) at different times(starting at 14:15 PM). A decay in the microbial load is observed overtime.

Example 4

The apparatus shown in FIG. 5 was operated using a single layer ofabsorbent material mounted therein. The electrolytic cell was activatedto generate an oxidant brine having a Redox potential of about 250 mVcirculating through the apparatus, and air samples were takenperiodically (at 12:00 PM, 12:40 PM and 12:50 PM) from both the incomingairflow and the outgoing airflow. FIGS. 11 a, 11 b and 11 c are photosof the TSA+5% SB settle plates, wherein the Petri dishes seen in theleft (designated 60 a, 60 b and 60 c) show the development of microbialcolonies originating from samples taken from the incoming, contaminatedair, while the Petri dish in the right (61 a, 61 b and 61 c) show thedevelopment of microbial colonies originating from samples taken fromthe treated, purified air. It is apparent from the photos that thepurification treatment according to the invention effectively reducesthe amount of microbial contamination in air.

A similar observation may be readily made on the basis of the SDA settleplates shown in FIGS. 12 a and 12 b. No bacterial colonies are visiblein the Petri dishes designated 71 a and 71 b, where samples taken fromthe outgoing, treated air were seeded. In contrast, visible colonieshave developed in the Petri dishes designated 70 a and 70 b, wheresamples taken from the incoming, pre-treated air were seeded.

Example 5

A similar experiment to that described in Example 4 was carried out, butthis time the apparatus was operated with two layers of cellulose fibersmounted therein. The Redox potential of the circulating brine was about230-250 mV. Air samples were taken periodically from both the incomingairflow and the outgoing airflow. It is apparent from FIGS. 13 a to 13Ethat the development of microbial colonies originating from samplestaken from the outgoing, treated airflow (the Petri dishes designated 91a-91 e) is considerably reduced in comparison to the correspondingsamples, taken from the incoming, non-treated airflow (the Petri dishesdesignated 90 a-90 e).

Example 6 The Efficacy of a Non-Electrolyzed Brine which Comprises aMixture of Halide Salts as a Disinfectant for Biologically ContaminatedAir

This example describes the results of tests that were carried out withthe apparatus illustrated in FIG. 5 in the intensive care childrendepartment of the Chaim Sheba Medical Center in Tel-Hashomer, Israel.The apparatus was connected to the air-conditioning system of thedepartment. The volume of the department space was approximately 1000m³; the rate of air suction of the air-conditioning system during thetests was about 2000 m³/hour while the rate of air suction through theapparatus of the invention was about 700 m³/hour. Fresh air from outsidewas fed into the apparatus, in a volumetric concentration of 15%relative to the total volume of air circulated.

The brine used was a Dead Sea brine, whose composition was identifiedabove, which brine further includes CaCl₂ in a concentration of 200g/lit. The brine was pumped at a rate of 10 m³/hour.

FIG. 14 is a bar diagram which describes the level of biologicalcontamination measured in the children's intensive care department atthe Sheba hospital (Israel) over a period of approximately two weeks. Inthe first week (the control week—between September 13 and September 20)high levels of contamination were measured. In the subsequent week theapparatus according to the invention was allowed to operate as describedabove, and a significant reduction in the biological contamination inthe air of said department was observed.

For the purpose of comparison, the quality of the air was also tested inan adjacent department—the interim care department (which was nottreated by means of the method of the invention). The measurements werecarried out twice a day both in the treated and untreated departmentsover a period of approximately two weeks, and the results collected areshown in FIG. 15, which is a bar diagram describing the level ofcontamination (CFU/m³) against time. It may be seen that the level ofcontamination in the treated department was consistently lower incomparison to the untreated department.

It is thus apparent from FIGS. 14 and 15 that a concentrated saltsolution which comprises a mixture of halide salts forms a particularlypowerful disinfectant.

1. A method for reducing biological contamination of indoor air, whichmethod comprises: providing a concentrated salt solution, which iseither (i) a brine capable of responding to aeration thereof by a rapidincrease of its Redox potential, wherein the rate of said increase isgreater than the rate of increase observed for a 45% (w/w) calciumchloride solution subjected to identical aeration conditions; or (ii) abrine which is passed through an electrolytic cell in order to raise itsRedox potential, wherein chlorine, hypochlorite and hypochlorous acidare generated in said brine; circulating said concentrated salt solutionthrough a treatment zone; causing a stream of biologically contaminatedair to flow through said treatment zone, such that said contaminated airis contacted with said salt solution in said treatment zone; wherein theRedox potential of the concentrated salt solution in the treatment zoneis from 200 mV to 370 mV; and withdrawing purified air from saidtreatment zone.
 2. A method according to claim 1, wherein theconcentrated salt solution is a brine capable of responding to aerationthereof by a rapid increase of its Redox potential, wherein the rate ofsaid increase is greater than the rate of increase observed for a 45%(w/w) calcium chloride solution subjected to identical aerationconditions.
 3. A method according to claim 2, wherein the brine containsone or more water soluble salts represented by the formulas MX, M₂X andMX₂, wherein X is selected from the group consisting of chloride,bromide, iodide, sulfate and nitrate anions, and M indicates a metalselected from the group consisting of sodium, potassium, calcium,magnesium and zinc, with the proviso than when X is chloride and M isother than zinc, then the solution comprises at least two water solublesalts.
 4. A method according to claim 2, wherein the brine comprises abromide salt and/or an iodide salt.
 5. A method according to claim 3,wherein the brine comprises zinc bromide, zinc chloride, or calciumbromide or a mixture thereof.
 6. A method according to claim 3, whereinthe brine comprises a mixture of at least one bromide salt and at leastone chloride salt of one or more of the following cations: Na⁺, K⁺, Mg²⁺and Ca²⁺.
 7. A method according to claim 6, wherein the totalconcentration of salts dissolved in the brine is in the range between 30and 40% by weight.
 8. A method according to claim 7, wherein the brinecomprises calcium chloride in a concentration effective of reducing theevaporation of water therefrom.
 9. A method according to claim 1,wherein the concentrated salt solution is a brine solution which ispassed through an electrolytic cell in order to raise its Redoxpotential.
 10. A method according to claim 9, wherein the brinecomprises sodium chloride in a concentration of between 30 and 40% byweight.
 11. A method according to claim 1, comprising passing the streamof contaminated air through or onto one or more surfaces provided withinthe treatment zone, wherein said one or more surfaces are wetted by theconcentrated salt solution circulating through said treatment zone,thereby increasing the liquid surface area in contact with the air. 12.A method according to claim 11, wherein the surfaces provided within thetreatment zone comprise one or more layers of absorbent material, smallglass balls, ceramic rings and plastic rings.
 13. A method according toclaim 12, wherein the surfaces are in the form of layers made ofabsorbent fibrous material, which layers are wetted by one or morestreams of the circulating salt solution, which streams flow in thetreatment zone in a direction which is opposite to the flow of thecontaminated air.
 14. A method for treating a biologically contaminatedair according to claim 1, which comprises causing a stream ofbiologically contaminated air to flow through a treatment zone,contacting said contaminated air with a concentrated salt solutioncirculating through said treatment zone and recovering a purified airtherefrom, wherein the circulation of said solution comprisesintroducing the solution into the treatment zone, collecting thesolution in a suitable vessel after it has been in contact with thecontaminated air, driving the solution from said vessel and raising theRedox potential of the solution by passing said solution through anelectrolytic cell, and reintroducing the solution into the treatmentzone, in order to purify biologically contaminated air flowingtherethrough.
 15. A method according to claim 14, wherein theconcentrated salt solution is circulated at ambient temperature.
 16. Amethod according to claim 1, wherein the indoor air which is purified isthe indoor air of a hospital, a poultry farm, a greenhouse, a tunnel ora train station.
 17. A method according to claim 1, wherein thereduction of the biological contamination comprises the reduction of thelevel of viruses in the indoor air.
 18. A method according to claim 1,which further comprises periodically or continuously measuring the Redoxpotential of the brine and adjusting the Redox potential of said brinebased on the measured value of the Redox potential.
 19. A methodaccording to claim 18, wherein the Redox potential of the brine isincreased by electrolyzing the brine.
 20. A method according to claim18, wherein the Redox potential of the brine is increased by enhancingthe rate of flow of air flowing through the treatment zone.
 21. A methodaccording to claim 18, wherein the Redox potential of the brine isdecreased by chemically treating the brine with one or moreoxidizer-scavenging compounds.
 22. A method according to claim 21,wherein the oxidizer-scavenging compound is a sulfur-based reducingagent selected from the group consisting of water soluble salts ofsulfite, bisulfite, thiosulfate, metabisulfite, hydrosulfite andmixtures thereof.